Anti-hydrogen bond between chloroform and fluorobenzene

Accurate theoretical calculations (ab initio MP2/6-31G* counterpoise-corrected gradient optimization, harmonic and anharmonic vibrational analysis) on the fluorobenzene ... chloroform complex predict a new type of bonding, termed the anti-hydrogen bond. This bond distinguishes itself by the contraction of the C-H bond of chloroform and a blue shift of the corresponding stretching frequency, i.e. features opposite to those characteristic for a hydrogen bond. The predicted blue shift was confirmed experimentally by double-resonance infrared ion-depletion spectroscopy. The calculated blue shift of the chloroform C-H stretching frequency (12 cm(-1)) agrees with the experimental value of 14 cm(-1). The anti-hydrogen bond originates from the dispersive interaction between molecules (contrary to the hydrogen bond which is of electrostatic origin). It plays a significant role in benzene-containing molecular clusters and is expected to be of consequence for the structure of biomolecules. (C) 1999 Elsevier Science B.V. All rights reserved.